This application is a 371 of PCT/EP99/04296 Jun. 21, 1999.
2-Nitro-5-(phenylthio)-anilines are useful intermediates for preparing herbicides (see EP-A 61 110) for preparing active compounds for the treatment of diseases caused by worms (see DE-A 2 332 398) and for preparing growth-promoting agents for domestic and working animals (see CH-A 619 458).
To prepare 2-nitro-5-(phenylthio)-anilines, 5-chloro-2-nitroanilines can be reacted with thiophenols for example by initially converting the thiophenpol, using an oily emulsion of sodium hydride in dimethylformamide, into the corresponding sodium thiophenolate and converting the latter, by addition of a 5-chloro-2-nitroaniline, into the product which is then precipitated using water (see DE-A 2 406 584).
According to DE-A 2 332 398 and DE-A 2 549 417, thiophenol and 5-chloro-2-nitroanilines were boiled under reflux in dimethylformamide in the presence of potassium carbonate for about 7 hours. After aqueous work-up and recrystallization, the product was obtained in yields of 77 to 88%. Using an almost identical procedure, but a reaction time of only 1 hour at 100xc2x0 C., a crude yield of 91% was obtained; however, the material still had to be recrystallized in order to achieve the required quality (see J. Med. Chem. 18, 1164 (1975)).
EP-A 61 110 likewise uses potassium carbonate. Here, 2-methyl-6-nitro-3-phenylthio-aniline was prepared in dimethyl sulfoxide at 110xc2x0 C., in a yield of 92%.
All these processes have the disadvantage that, for product isolation, the reaction mixtures, which comprise dipolar aprotic solvents, have to be subjected to aqueous work-up. This gives rise to waste water which carries a high load of organic solvents and has to be disposed of. Furthermore, the yield and/or the purity of the product obtained is frequently unsatisfactory.
For carrying out the reaction in ethanol under reflux conditions, potassium hydroxide was employed as base (See CH-A 619 458 and Arch. Pharm. 316, 638 (1983)). After cooling of the reaction mixture, the product was filtered off and then recrystallized, likewise giving unsatisfactory yields of from 80 to 83%.
According to DE-A 4 202 262, 5-chloro-2-nitroaniline was prepared with thiophenol in a two-phase system of aqueous sodium hydroxide solution and toluene with participation of tetrabutylammonium bromide, in a yield of 98%. Other phase-transfer catalysts gave poorer yields. This process generates waste water loaded with the phase-transfer catalysts, some of which are toxic, and the waste water is very difficult to purify, owing to the presence of the phase-transfer catalysts.
Accordingly, there is still a need for a process for preparing 2-nitro-5-phenyl-thio-anilines which can be carried out giving good yields, but which does not cause any particular ecological problems.
Accordingly, the present invention provides a process for preparing 2-nitro-5-(phenylthio)-anilines of the formula (I) 
in which
R1 represents hydrogen, C1-C8-alkyl, C3-C8-cycloalkyl, C1-C8-alkoxy or halogen and
R2 represents hydrogen, C1-C8-alkyl, C3-C8-cycloalkyl, C1-C8-alkoxy, halogen or optionally C1-C8-alkyl-, C1-C8-alkoxy- or halogen-substituted C6-C10-aryl,
by reacting 5-chloro-2-nitroanilines of the formula (II) 
in which
R1 is as defined under formula (I)
with thiophenols of the formula (III) 
in which
R2 is as defined under formula (I),
characterized in that a 5-chloro-2-nitroaniline of the formula (II) is reacted in a solvent with a thiophenol of the formula (III) and ammonia.
In the formulae (I) and (II), R1 preferably represents hydrogen, C1-C4-alkyl or chorine, particularly preferably hydrogen, methyl or chlorine.
The 5-chloro-2-nitroanilines of the formula (II) and thiophenols of the formula (III) which can be used for the process according to the invention can be obtained in a known manner or analogously thereto, if they are not commercially available.
In a preferred embodiment of the process according to the invention, the 5-chloro-2-nitroaniline is question is prepared by chlorine-amine exchange from the corresponding 2,4-dichloronitrobenzene, using ammonia (see, for example, J. Med. Chem. 35, 4455 (1992) and DE-A 3 431 827)), and the process according to the invention is then carried out without intermediate isolation of the 5-chloro-2-nitroaniline formed, by adding the thiophenol and, if required, additional ammonia, in the same reaction vessel. In principle, the order in which the reactants are added is not critical for the process according to the invention. Preferably, the 5-chloro-2-nitrolaniline of the formula (II), dissolved in a solvent, is mixed with ammonia, and a thiophenol of the formula (III) is then added. The reactants can also be added in any other order. They can also be metered in simultaneously into the reaction mixture.
In the formulae (I) and (III), R2 preferably represents hydrogen or C1-C4-alkyl, particularly preferably hydrogen.
Suitable solvents for the process according to the invention are virtually all customary solvents, for example inorganic solvents, such as water or anhydrous ammonia, and organic solvents, such as alcohols, ethers, hydrocarbons, aromatics, chloroaromatics and dipolar aprotic solvents. Specific examples of organic solvents which may be mentioned are: methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, sec-butanol, tert-butanol, glycol, diethyl ether, methyl tert-butyl ether, tetrahydrofuran, dimethylformamide, N-methylpyrrolidone, toluene and chlorobenzene. Preference is given to nonpolar and low-polarity solvents, such as alcohols, ethers, hydrocarbons, aromatics and chloroaromatics, in particular methanol, isopropanol, isobutanol, toluene and chlorobenzene.
The use of phase-transfer catalysts is not required.
The ammonia used can be of technical grade.
Based on 1 mole of the 5-chloro-2-nitroaniline used, it is possible to employ, for example, from 0.5 to 2 mol, in particular from 1 to 1.2 mol, of a thiophenol and from 1 to 30 mol, in particular from 2 to 15 mol, of ammonia.
The process according to the invention can be carried out, for example, at temperatures in the range from 20 to 140xc2x0 C. The process is preferably carried out at from 40 to 100xc2x0 C. During the practice of the process according to the invention pressures in the range from 1 to 20 bar, for example, may be present. The process is preferably carried out at from 3 to 12 bar, in particular under the autogenous pressure that becomes established in a sealed reaction vessel under the reaction conditions used. In particular towards the end of the reaction, it is advantageous to keep the temperature below 80xc2x0 C. and the pressure below 10 bar.
The process according to the invention can be carried out, for example, by initially charging the 5-chloro-2-nitroaniline of the formula (II) together with the solvent in an autoclave, then heating the mixture to the desired reaction temperature, then adding ammonia and then metering in the thiophenol of the formula (III), for example in the course of from 0.5 to 5 hours. It is advantageous to meter in additional ammonia during the metered addition of the thiophenol, for example such that the pressure which prevailed prior to the metered addition of the thiophenol is maintained, within a range of xc2x120%. After the metered addition of the thiophenol has ended, the reaction mixture may be stirred for some extra time, for example for from 1 to 20 hours, under the abovementioned pressure and temperature conditions, in particular at temperatures below 80xc2x0 C.
The work-up of the reaction mixture which is then present is simple. Frequently, in particular when non-polar or low-polarity solvents are used, a suspension which contains the 2-nitro-5-(phenylthio)-aniline of the formula (I) which has been prepared in solid form is present after cooling of the autoclave. In this case, it is generally sufficient to filter off the reaction mixture and to wash the solid residue, for example with water. If desired, ammonia can be recovered from the ammonium chloride that is formed, by adding a strong base, for example aqueous sodium hydroxide solution. If dipolar aprotic solvents are used, it is advantageous to add water to the reaction mixture prior to the isolation of the product that has been prepared.
The process according to the invention affords 2-nitro-5-(phenylthio)-anilines of the formula (I) generally in yields of more than 88% of theory and purities of more than 90% by weight. Frequently, the yields are above 92% of theory, for example above 95% of theory, and the purities frequently exceed 94% by weight.
It is extremely surprising that, using the process according to the invention, such good results can be obtained in a simple manner, without any particular associated ecological problems, since ammonia itself is a good nucleophile and capable of replacing chlorine bound to aromatic compounds by the amino group. Surprisingly, however, virtually no such reaction occurs with the compounds of the formula (II) used. In contrast, in the process according to the invention, the chlorine which is present in the compounds of the formula (II) is, in an unexpected manner and despite the presence of ammonia, selectively replaced by the thiophenol radical.
The ecology is particularly favourable when nonpolar or low-polarity solvents are used, since only small amounts of these pass over into the waste water. Hitherto, however, the use of dipolar aprotic solvents or the use of phase-transfer catalysts (in combination with low-polarity solvents) has been considered to be indispensable. However, in the presence of phase-transfer catalysts large amounts of dipolar aprotic solvents and low-polarity solvents end up in the waste water.